Consumer packages of a non-returnable character are manufactured at present with the help of modern, high-capacity machines of the type which from a web or from prefabricated blanks of a packing material form, fill and close the packages. The machines manufacture packages from a web by first converting the web to a tube by durably joining together the longitudinal edge zones in a strong overlap joint. The tube formed is then filled with the intended contents and divided into individual, filled packing units through repeated transverse sealings of the tube across the longitudinal axis of the tube below the actual contents level, whereafter the packing units are separated from one another by means of transverse cuts in the transverse sealings made, and, possibly after a final shaping, discharged from the machine as finished packages.
A very large group of these so-called non-returnable packages is manufactured from a material comprising a carrier layer of paper or cardboard and outer and inner coatings of plastics, in particular thermoplastics, which beside making the packages liquid-tight also may be used for performing the aforementioned sealings through so-called heat-sealing during the manufacture of the packages.
With the help of packing machines of the above-described type, it is also known to make so-called aseptic packages for certain types of sensitive, liquid foodstuffs, e.g. milk, in order to prolong the keeping properties of the contents. The aseptic machines operate in principle in the same manner as the machines described earlier, but with the important difference that the manufacture of the packages is carried out under aseptic conditions which means that the contents as well as the packing material have to be sterile, and likewise, the atmosphere in the machine where the tubes are formed and filled must be sterile. The sterile atmosphere in the machine is obtained by maintaining a certain pressure of sterile gas, usually superheated sterile air, inside the tubes as well as in the close environment of the tubes. As a result, leakage of polluted, non-sterile air from the outer environment of the machine is prevented. The sterile contents usually are obtained by subjecting the contents prior to filling to a heat treatment whereby the contents for a certain period are heated to, and held at, a sufficiently high temperature in order to eliminate harmful micro-organisms. It has not been too difficult to fulfil these two sterility requirements, but it has been found more difficult, with the methods available up to now, to provide a simple, effective sterilization of the weblike packing material.
The sterilization of the packing material web is carried out by passing the web prior to conversion to tubular shape through a bath of chemical sterilizing agent, usually a 10-35% hydrogen peroxide solution which is made to moisten the packing material, whereupon the surplus of liquid is removed from the web by means of calender cylinders. Any sterilizing agent remaining on the web is removed, after conversion of the web to a tube, by a heating arrangement which heats the material tube to such a degree that the agent is evaporated and driven off through the upper, open end of the tube.
In accordance with another known method, the packing material web is passed through a chamber containing heated, gaseous sterilizing agent, preferably a mixture of hydrogen peroxide and steam, to absorb hydrogen peroxide through condensation on the material web. In this known method too the remaining sterilizing agent is removed by evaporation.
Even though the known methods described here, which use liquid sterilizing agent either directly or indirectly through condensation, function well for material webs with plain, uniform surfaces, it has proved more difficult to achieve an effective sterilization of packing material webs with surface irregularities, e.g. tear strips (so-called pull-tabs) sealed over prepared emptying openings. This is due, at least partly, to the material web being in contact with the sterilizing agent during a time which is too short to allow the sterilizing agent to penetrate, and act in, the less readily accessible spaces of such irregularities. Another problem, which is also connected with using the sterilizing agent in liquid form, and which becomes particularly manifest when the web is passed through a bath, is the difficulty of preventing so-called edge absorption of the sterilizing agent in web portions with freely exposed fibre layers, e.g. in the area along longitudinal cutting edges of the web which easily absorb moisture.
It is known that a mixture of hydrogen peroxide and water in gas form has a sterilizing effect which increases with rising temperature, and it is known, moreover, that gas, by contrast to liquid, can easily penetrate into less readily accessible areas of the type which occur on material webs with surface irregularities, and a natural and obvious solution of the problems which are inherent in the known methods described should be, therefore, to substitute the sterilizing agent in liquid form by a corresponding sterilizing agent in gas form and carry out the sterilization exclusively in the gas phase, that is to say without condensation.
Notwithstanding the realization that an effective sterilization can be carried out with the help of a gaseous sterilizing agent, e.g. water/hydrogen peroxide-vapour, regardless of the surface quality of the material web, it has been difficult up to now to utilize that method.